Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. This former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art FIG. 1 (adapted from U.S. Pat. No. 7,163,278) as an example of a conventional thermal inkjet drop on demand drop ejector configuration. Partition walls 20 are formed on a base plate 10 and define pressure chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32, each nozzle 32 being disposed over a corresponding pressure chamber 22. Ink enters pressure chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink inlets 24, as indicated by the arrows in FIG. 1. A heater 35, which functions as the actuator, is formed on the surface of the base plate 10 within each pressure chamber 22. Heater 35 is configured to selectively pressurize the pressure chamber 22 by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle 32 when an energizing pulse of appropriate amplitude and duration is provided.
Other types of actuators that use resistive heaters to selectively pressurize the pressure chamber for drop ejection include thermal actuators that have a multi-layer cantilevered element that is caused to rapidly bend toward the nozzle when the resistive heater layer is pulsed. Less heating of the ink is required than for thermal inkjet, where the ink is locally vaporized to provide the ejection pressure.
Drop ejectors in drop-on-demand inkjet printheads work well within a given temperature range. Printhead temperature can vary due to variation in ambient temperature as well as to temperature rise associated with the energy dissipated on the printhead during operation. A known problem in drop-on-demand inkjet printing is the degradation in output print quality due to temperature-related changes in the volume of ink that is ejected. One reason why the size of ejected drops increases with temperature of the printhead is that ink viscosity decreases with increased temperature. In addition, for thermal inkjet printheads, the amount of ink that is vaporized by a resistive heater during a printing pulse increases with increased printhead temperature. Although a significant portion of the heat is carried off by the ejected ink drops, some of the heat remains in the printhead and results in an increased temperature. At sufficiently high temperature the drop ejection can become unreliable, resulting in missing dots in the printed image.
Various printhead temperature control and pulse waveform control systems and methods are known in the prior art for sensing inkjet printhead temperature and using sensed temperature signals to compensate for temperature fluctuations. The approach in printhead temperature control is to keep the printhead within a narrow temperature range by auxiliary heating or cooling for example. In pulse waveform control the approach is to tailor the pulses that are provided to the resistive heaters in order to compensate for temperature changes on the printhead so that the drop volume remains substantially constant. In both approaches it is important to have an accurate measurement of temperature in the vicinity of the drop ejectors.
U.S. Pat. No. 4,910,528 discloses an analog temperature sensing system where a thin film thermal sensing resistor (i.e. a thermistor) is formed on the same substrate as the drop ejectors in order to provide a temperature measurement that corresponds closely to the temperature of the drop ejector substrate. It is disclosed that preferably four leads are attached to the thermistor where two of the leads provide a current and the other two leads are used to output the voltage drop across the thermistor.
U.S. Pat. No. 4,791,435 discloses a thermal inkjet printhead temperature control system that regulates the temperature of a printhead using a temperature sensing device and a heating component. The temperature sensing device includes either a collection of transducers or a single thermistor located on the drop ejector substrate or on a printed circuit board to which the printhead is attached.
U.S. Pat. No. 5,075,690 discloses an analog temperature sensor that is formed on the drop ejector substrate and extends along the length of the array of drop ejectors. Recognizing that manufacturing variations in the thermistor can result in large inaccuracies in temperature measurement, a factory calibration is disclosed where a resistor in series with the thermistor is trimmed, for example by laser trimming, while the printhead is held at a set point temperature. Other methods of factory calibration of the thermistor on an inkjet printhead are described in U.S. Pat. No. 5,881,451 and U.S. Pat. No. 7,572,051.
The analog measurement of a temperature sensor such as a thermistor can be converted to a digital signal by an analog to digital converter for use in control circuitry as disclosed in U.S. Pat. No. 6,302,507 and in U.S. Pat. No. 6,322,189. Another alternative for providing a digital signal is to provide temperature controlled oscillator circuitry on the drop ejector substrate. Temperature controlled oscillators are described in U.S. Pat. No. 5,388,134 and typically include a thermistor as the temperature sensitive element. The number of counts recorded by a counter during a given time interval (i.e. the frequency of the oscillator signal) changes approximately linearly with temperature. Various implementations of temperature controlled oscillators on inkjet printheads are disclosed in U.S. Pat. No. 5,745,130, U.S. Pat. No. 6,037,831, U.S. Pat. No. 6,278,468, and U.S. Pat. No. 8,419,158.
In some of the prior art references listed above, such as U.S. Pat. No. 5,745,130, U.S. Pat. No. 6,302,507, U.S. Pat. No. 7,572,051 and U.S. Pat. No. 8,419,158 a plurality of small temperature sensors are located in various parts of the drop ejector substrate either for obtaining an average temperature measurement on the drop ejector substrate or for independently measuring the temperature in different locations on the drop ejector substrate. Typically for measuring temperature in different locations using temperature sensors formed on the drop ejector substrate, additional output leads are required as shown in prior art FIG. 2 adapted from U.S. Pat. No. 5,467,113. Drop ejector array substrate 1 includes temperature sensors 2a and 2b, an array of drop ejection heaters 3, and warming heaters 8a and 8b for maintaining the temperature within a predetermined range. Warming heaters 8a and 8b are individually and independently controlled in response to the outputs from the temperature sensors 2a and 2b respectively Temperature sensors 2a and 2b are connected to different output pads (not labeled) that are located near the bottom edge of the drop ejector array substrate.
As indicated above, drop volume tends to increase with increased temperature of the printhead and ink. It is also known that drop volume can be affected by the pulse waveform. As disclosed in U.S. Pat. No. 4,982,199, ink in the vicinity of the nozzle of a drop ejector can be pre-warmed by pulsing the resistive heater using one or more pulses that have insufficient energy to form a vapor bubble of ink prior to the firing pulse that forms the vapor bubble. By pre-warming the ink, more of the ink in the nozzle region is brought to the vaporization temperature by the firing pulse before the transfer of heat to the ink from the resistive heater is interrupted by the formation of the vapor bubble. Vaporizing more of the ink forms a larger bubble, which provides the power for ejecting a larger drop of ink. Prior art FIGS. 3A to 3G are a copy of FIGS. 3A to 3G of U.S. Pat. No. 4,892,199. In each of FIGS. 3A to 3G the dashed line represents a pulsing level that is sufficient to form a vapor bubble. Three firing pulses (unlabeled) are shown in FIG. 3A to 3G that extend beyond the dashed line. The firing pulses are preceded by pre-warming pulses (unlabeled) of different shapes, widths, numbers and amplitudes. U.S. Pat. No. 4,982,199 contemplated the use of pre-warming pulses for use in gray-scaling rather than for compensation of drop volume for temperature variation.
U.S. Pat. No. 5,036,377 disclosed attaching a temperature sensor to a surface of the drop ejector substrate. The resistive heaters on the drop ejector substrate are connected to drivers that are not on the drop ejector substrate. Temperature signals from the temperature sensor are sent to a controller, and the controller enables actuation of selected resistive heaters through the drivers using packets of electrical pulses. A digital clock signal is also provided to the controller. It is disclosed that pulse widths, idle times between pulses or number of pulses per packet can be increased or decreased by one or more clock units to change the pulse waveform in order to control drop volume in response to the temperature measured by the temperature sensor according to a look-up table that provides data to the controller. U.S. Pat. No. 5,917,509 discloses one or more precursor pulses (or warming pulses) that are applied to the resistive heater for warming the ink nearby, followed by a print pulse that causes a drop of ink to be ejected.
Despite the previous advances in temperature sensing as well as temperature control and drop volume control on inkjet printheads, what is still needed are printing system designs and printing methods that provide individual temperature sensing and corresponding pulse waveform compensation for different locations on a drop ejector array substrate. In addition, it is desirable to provide drop ejector arrays having a small number of input/output connections, while still providing drop volume control for drop ejectors in different locations on a drop ejector array substrate that can be at different temperatures.